Chromium (Cr) is an industrially important element, necessary for chrome plating and the production of stainless steel. The only source of metallic chromium that exists is chromite ore (Cr2O3), which commonly occurs as chromite (FeCr2O4) where iron in the formula can be substituted by magnesium and chromium by both aluminum and ferric iron. Ferrochrome smelting using conventional carbothermic methods is an energy-intensive process, requiring energy inputs up to 4.6 MWh for each tonne of ferrochrome produced.
“Prereduction” (direct reduction of the chromite ore before smelting) can allow reduction and metallization to occur at lower temperatures, thus requiring less energy. In this context, “reduction” and “prereduction” refer to a chemical process wherein oxygen is removed from one reactant (here, the chromite ore) and taken up by another reactant (referred to as the “reductant”). Hence, the oxidation states of the constituents of one reactant (the chromite here) is “reduced”.
Although prereduction enables lower temperatures, the process occurs in solid-state (meaning that both the chromite ore and the reductant are in solid form). Solid-state reactions are kinetically slow and rarely result in completely metallized chromite ore. The greater the metallization during prereduction, the lower the energy requirements can be, and the greater the energy savings.
Additionally, low ash coke, the most common reductant source in the smelting process, is expensive in itself. A method that increases metallization before smelting without requiring a high quality reductant would be more cost-effective than traditional smelting processes.
It is common to add fluxing agents to the reduction furnace, to improve the metallization rate. These fluxing agents enhance the formation of a liquid slag layer in the chromite ore and allow greater metallization at lower temperatures. Several kinds of fluxing agent have been considered in the prior art, including alkali salts, borates, carbonates and silicates. Addition of these fluxes decreases the melting temperature of refractory oxides namely, MgO and Al2O3. This has enabled chromite reduction to occur effectively even at temperatures under 1400° C. (as compared to reduction temperatures of up to 2000° C. for smelting).
However, not all of these fluxes are easily available. They may be expensive, uncommon, or both. Thus, there would be a benefit to the use of an additive that is not only useful, but also widely available and cost-effective. Preferably, such an additive would allow for chromite reduction at even lower temperatures.